1. Field of the Invention
This invention relates to the field of logging of oil, gas and mineral wells. More particularly, improvements relating to real-time signal processing and pulse compression for multi-frequency array induction logging are disclosed.
2. Description of Related Art
Induction logging tools are instruments used in logging operations in boreholes that are drilled into underground rock formations in the search for oil, gas or minerals. These tools measure the electrical conductivity of rock formations to determine the presence and the amount of desired minerals in a potential pay zone. Oil and natural gas cause the rock to have a lower than usual conductivity because these fluids are electrically non-conducting and they displace connate saline water. Induction logging tools ideally provide accurate quantitative measures of the fractional saturation of oil or gas in the pay-zone.
Induction logging tools employ sensor arrays that map the rock conductivity at various radial distances from the borehole, so that the influence of invasion of borehole fluids may be reduced. These tools operate on the principle of induced eddy currents, which are substantially proportional to conductivity, and which may be excited and detected using sensitive coils. Tools that are known in the art use arrays of coils that provide capabilities to sense conductivity to different radial distances from a wellbore.
U.S. Pat. No. 5,157,605 discloses an induction logging method and apparatus for operating an induction sound at a plurality of frequencies simultaneously. A plurality of two-coil receiver arrays are used. U.S. Pat. No. 5,548,219 discloses a system for generating multiple frequencies for heterodyne measurement system for use in logging. U.S. Pat. No. 7,183,771 discloses a device comprising a circuit for injecting a calibration signal into the receivers to obtain measurements free from errors introduced by the receiving elements of the system.
Despite many advances in induction logging tool technology, several electronics-related problems remain to be solved. For example, electronics signal fidelity issues are still observed in the noisy environment of downhole logging, especially when highly conductive formations are present. Further problems for induction logging tools are caused by the “skin effect.” Skin effect causes a loss of proportionality between a received signal and formation conductivity, thereby making interpretation of signals from induction logging tools more complex. Conversely, very low conductivity rocks present accuracy problems for induction logging tools due to low signal-to-noise ratios. These problems are made more challenging when the beds or rock formations of interest are relatively thin.
Attempts to resolve these problems have exhibited shortcomings, including a high cost/benefit ratio. What is needed is a cost-effective, robust, electronics subsystem with a design centered on noise minimization/cancellation resulting in improvements in data signal fidelity and more accurate logging results.